This invention relates to a flow measurement correction for an exhaust flow measurement system.
A prior art exhaust flow measurement system 10 is shown in FIG. 1 with an exhaust F flowing through an exhaust flow tube 12. The system 10 includes a transducer 14 that measures the flow of exhaust F past the transducer 14 to calculate the flow rate, as is well known in the art. In the example shown, the transducer 14 includes first 16 and second 18 transducer portions that produce a signal that is sent to a signal processing unit 20 indicative of exhaust flow rate.
Inherent in the exhaust flow measurement system 10 are pulsations in the exhaust flow that undesirably change the rate of exhaust flow F past the transducer 14. A graph of flow rate versus time is shown in FIG. 2. The signal 22 is indicative of a flow rate having areas of noise 24, which results in undesirable inaccuracies in flow rate measurement that comprise the integrity of the test. While software is used to electronically filter much of the noise from the signal, significant inaccuracies still occur at the harmonics of the flow meter sampling frequency, also referred to aliasing.
A graph of a software-filtered signal 26 illustrated as amplitude versus frequency is shown in FIG. 3. As can be seen from the graph, the inaccuracies due to sampling rate 28 manifest themselves as large peaks at the harmonic frequencies that repeat at a particular frequency interval. The aliasing error manifests itself as the exhaust flow is read as a low frequency thereby “fooling” the software filter, which filters out higher frequencies. For the example shown, the sampling rate of the transducer is 40 milliseconds resulting in an aliasing error every odd multiple of 25 Hz, which is particularly problematic since the idle of some test engine are at or near the same frequency. Of course, a different sampling rate will produce an aliasing error at other frequencies, as can be appreciated by the equation accompanying FIG. 3.
Since the software filter is ineffective to eliminate inaccuracies due to aliasing, other means must be employed to eliminate the aliasing error.